Multiaxial fatigue and life prediction of elastomeric components

Elastomeric components have wide usage in many industries. The typical service loading for most of these components is variable amplitude and multiaxial. In this study a general methodology for life prediction of elastomeric components under these typical loading conditions was developed and illustrated for a passenger vehicle cradle mount. Crack initiation life prediction was performed using different damage criteria. The methodology was validated with component testing under different loading conditions including constant and variable amplitude in-phase and out-of-phase axial–torsion experiments. The optimum method for crack initiation life prediction for complex multiaxial variable amplitude loading was found to be a critical plane approach based on maximum normal strain plane and damage quantification by cracking energy density on that plane. Rainflow cycle counting method and Miner’s linear damage rule were used for predicting fatigue life under variable amplitude loadings. The fracture mechanics approach was used for total fatigue life prediction of the component based on specimen crack growth data and FE simulation results. Total fatigue life prediction results showed good agreement with experiments for all of the loading conditions considered.     

Fatigue life prediction of a rubber mount based on test of material properties and finite element analysis

Failure analysis and fatigue life prediction are very important in the design procedure to assure the safety and reliability of rubber components. The fatigue life of a rubber mount was predicted by combining test of material properties and finite element analysis (FEA). The natural rubber material material’s fatigue life equation was acquired based on uniaxial tensile test and fatigue life tests of the natural rubber. The strain distribution contours and the maximum total principal strains of the rubber mount at different loads in the x and y directions were obtained using finite element analysis method. The critical region cracks prone to arise were obtained and analyzed. Then the maximum total principal strain was used as the fatigue parameter, which was substituted into the natural rubber’s fatigue life equation, to predict the fatigue life of the rubber mount. Finally, fatigue lives of the rubber mount at different loads were measured on a fatigue test rig to validate the accuracy of the fatigue life prediction method. The test results imply that the fatigue lives predicted agree well with the test results.     

Damage-based modification for fatigue life prediction under non-proportional loadings

This paper proposes a new fatigue model based on virtual strain energy to predict fatigue life under both proportional and non-proportional loadings for different materials including, 1045 Steel, 30CrNiMo8HH, Titanium TC4, and AZ31B magnesium. The results were strongly correlated with experimental results available in the literature. In addition, two damage-based modifications for fatigue life prediction under non-proportional loadings are studied. These modifications are then applied to the fatigue parameters including Smith–Watson–Topper, Fatemi–Socie, maximum shear strain, and the proposed parameter for fatigue life predictions of the studied materials. The results show considering these modifications significantly improves the accuracy of the models.     

Fatigue crack initiation life prediction for aluminium alloy 7075 using crystal plasticity finite element simulations

An experimentally-validated approach for predicting fatigue crack initiation life of polycrystalline metals is developed based on crystal plasticity finite element (CPFE) simulations. In this approach, the microstructure used in the simulations possesses statistically the same grain size and crystallographic orientations as those obtained from electron back-scatter diffraction experiments. A backstress model is incorporated into the CP constitutive model to describe the mechanical behaviour of aluminium alloy (AA) 7075 under cyclic loading. The key variables of the prediction model, the energy efficiency factor and plastic strain energy density, are calibrated using a fatigue test on a round-notched AA7075 specimen at room temperature. The proposed approach is then validated by using another fatigue test to predict 69.1–87.3% of the experimentally measured fatigue crack initiation life. The effects of the microstructure and texture on the energy efficiency factor and fatigue life prediction are quantitatively determined. It is shown that for a given range of energy efficiency factors a similar range of life prediction is obtained. Since the proposed approach considers the heterogeneity of the microstructure, it can well capture the grain scale deformation localisation and therefore improve the precision of fatigue life prediction.     

Numerical analysis of wheel cornering fatigue tests

In automotive engineering, the wheels are one of the most critical components and their function is of vital importance n human safety. The cornering fatigue test is one of the traditional durability tests for wheel prototype verification. In this paper, a bi-axial load–notch strain approximation for proportional loading is proposed to estimate the fatigue life of a passenger car wheel during the cornering fatigue test under plane stress conditions. The elasto-plastic strain components are calculated analytically using the total deformation theory of plasticity. The input for the load–notch strain analysis is the measured or calculated plastic strain state at the notch together with the materials stabilised cyclic stress–strain curve evaluated with unnotched tension specimens. The damage accumulation is based on the Palmgren–Miner rule. The methodology is implemented in a program called “Metal Fatigue Prediction and Analysis” (MFPA). The life prediction of a passenger car wheel during the cornering fatigue test is performed. The results of the analysis is compared with two cornering tests on the same design. The result is very encouraging and the application of the developed MFPA program provides time and the cost savings in the analysis of wheel cornering fatigue tests.     

Stepped‐isothermal fatigue analysis of engine piston

Stepped‐isothermal fatigue failure is the main failure mechanism of modern engine pistons under bench reliability test condition. This paper presents a methodology for stepped‐isothermal fatigue analysis of engine pistons, which consists of a fatigue criterion, evaluation of temperature and stress distribution by finite element analysis and the final life prediction. The major character of the methodology is the fatigue definition of engine pistons with respect to engine load change cycle and a damage‐based fatigue criterion accounting for the nonlinear creep–fatigue damage. Taking as an example, the fatigue life of an engine piston was predicted by the proposed analysis procedures. The analysis results showed that the most critical area was located in the throat edge. Moreover, the proposed methodology can give a relatively accurate and reasonable life prediction for an engine piston under the loading condition of bench reliability test, with a benefit of decreasing the needed component's reliability tests and design time.     

A fatigue model for sensitive materials to non-proportional loadings

The present study proposes a novel fatigue model based on virtual strain energy. This model separates loading paths based on their non-proportionality where directly takes into account the loading in fatigue life prediction. The proposed fatigue model is expressed in two tension-based and shear-based equations for two tensile and shear cracking failure modes. The model was validated against several experimental datasets available in the literature. In addition, obtained results were compared to predicted lives through some well-known fatigue models comprising maximum shear strain, Smith–Watson–Topper, and Fatemi–Socie. The results were strongly correlated with the experimental data indicating accuracy of the model.     

考虑棘轮效应的疲劳寿命预测方法

多轴棘轮加载时轴向加载的恒定应力、剪切应变幅对轴向棘轮应变和疲劳寿命有很大的影响.考虑棘轮效应影响的Coffin模型将棘轮效应与循环部分相结合来计算疲劳寿命,预测结果较好,绝大部分预测结果分布在2倍分散带内.     

EIFS-based crack growth fatigue life prediction of pitting-corroded test specimens

Corrosive environment causes corrosion pits at material surface and reduces the fatigue strength significantly. Fatigue crack usually initiates at and propagates from these locations. In this paper, a general methodology for fatigue life prediction for corroded specimens is proposed. The proposed methodology combines an asymptotic stress intensity factor solution and a power law corrosion pit growth function for fatigue life prediction of corroded specimens. First, a previously developed asymptotic interpolation method is proposed to calculate the stress intensity factor (SIF) for the crack at notch roots. Next, a growing semi-circular notch is assumed to exist on the specimen’s surface under corrosive environments. The notch growth rate is different under different corrosion conditions and is assumed to be a power function. Fatigue life can be predicted using the crack growth analysis assuming a crack propagating from the notch root. Plasticity correction is included into the proposed methodology for medium-to-low cycle fatigue analysis. The proposed methodology is validated using experimental fatigue life testing data of aluminum alloys and steels. Very good agreement is observed between experimental observations and model predictions.     

Damage tolerance reliability analysis of automotive spot-welded joints

This paper develops a damage tolerance reliability analysis methodology for automotive spot-welded joints under multi-axial and variable amplitude loading history. The total fatigue life of a spot weld is divided into two parts, crack initiation and crack propagation. The multi-axial loading history is obtained from transient response finite element analysis of a vehicle model. A three-dimensional finite element model of a simplified joint with four spot welds is developed for static stress/strain analysis. A probabilistic Miner's rule is combined with a randomized strain-life curve family and the stress/strain analysis result to develop a strain-based probabilistic fatigue crack initiation life prediction for spot welds. Afterwards, the fatigue crack inside the base material sheet is modeled as a surface crack. Then a probabilistic crack growth model is combined with the stress analysis result to develop a probabilistic fatigue crack growth life prediction for spot welds. Both methods are implemented with MSC/NASTRAN and MSC/FATIGUE software, and are useful for reliability assessment of automotive spot-welded joints against fatigue and fracture.     

Hip stem fatigue test prediction

The accuracy of fatigue test prediction methods for the standard fatigue testing of hip stems was evaluated against the experimental results of static and fatigue tests. Axial unnotched strain-controlled material fatigue tests provided the required cyclic material properties. Finite element analysis of the hip stems predicted a maximum tensile stress to within 3–7% of strain gauge measurements. The four methods investigated accurately predicted hip stem fatigue strength at 5 million cycles (?1% to ?9% errors). The strain–life methods successfully predicted fatigue life (factors 1/7.0–9.2 of the test) at high and low stress amplitudes of 352 and 315 MPa, respectively. The classical stress–life method was only accurate (factor 1/1.9) for the low stress level. The current study has demonstrated that fatigue test prediction methods can be applied with confidence to support standard fatigue testing of hip stems. Further studies can expand the understanding of these methods and their clinical relevance by investigating effects due to variable amplitude loading and environment.     

Failure analysis of an automobile damper spring tower

The cause of a passenger car’s damper spring tower early failure is investigated in this paper. Inspection of the road surface, tire inflation pressure, suspension, and service load are firstly done in order to determine the further test procedures and analysis methods. The static stress of the spring tower caused by the body weight is calculated by finite element model. Public road tests with an equipped car are carried out to simulate the real usage by the customers. With the measured strain signals of different test conditions and local strain–life method, fatigue life prediction is made. The calculated fatigue life coincides with the actual failure mileage, and it turns out that the broken spring damper causes the early failure of the spring tower. It is suggested that more emphasis should be taken on the durability design and test of the spring damper.     

The fatigue damage development in a cast Al–Si–Cu alloy

The experimental identification of fatigue damage mechanisms and evaluation of their development rate, based on changes in material respond on cycle loading, has been presented in the work. The research has been conducted on hyper-eutectic cast alloy AlSi8Cu3. The microstructure and fracture analyses were performed. The high cycle fatigue tests were conducted with frequency of 20 Hz under constant nominal stress amplitude with monitoring the strain response of material during the test. The ratcheting was found as the main mechanism of the fatigue damage. It was established that the linear fatigue accumulation law should not be used for fatigue life prediction in case of the tested cast aluminum alloy.     

Torsional fatigue with axial constant stress for Sn–3Ag–0.5Cu lead-free solder

A series of multiaxial ratcheting–fatigue interaction tests have been carried out on Sn–3Ag–0.5Cu lead-free solder specimens. All tests were conducted under cyclic shear strain with the constant axial stress at the room temperature with the shear strain rate of 5 × 10⁻³ s⁻¹. It was found that the ratcheting strain increased with increasing axial stress and shear strain amplitude while the fatigue life decreased at the same time. The ratcheting strain rate was linear with axial stress in double logarithmic coordinate. The Ohno–Wang II constitutive model was employed to simulate the stress–strain responses. Several fatigue life prediction models were applied to predict the multiaxial ratcheting–fatigue life of the Sn–3Ag–0.5Cu lead-free solder. The Gao–Chen model which adopted the maximum shear strain and the ratcheting strain rate as the damage parameter predicted the multiaxial ratcheting fatigue life well.     

High temperature low cycle fatigue behaviour of UDIMET 720 Li superalloy

The low cycle fatigue behaviour of UDIMET 720 Li has been investigated at 700 °C in vacuum and air environments under strain control tests. Hold times (HTs) have been introduced at peak tensile strain in order to determine the effects of creep and fatigue interactions. The fatigue life of the alloy is practically independent of HT in both environments at high strain ranges while HT has beneficial effect at low strain ranges. The main factor that adversely affects the life of the alloy is oxidation, as the fatigue life is reduced in air compared to vacuum especially at high strain ranges. The increase in fatigue due to HT at lower strain ranges is attributed to stress relaxation. A life prediction model has shown that nucleation occurs after few cycles at high strain ranges whereas a large proportion of the fatigue life is consumed in the nucleation process at low strain ranges.     

Global‐local fatigue assessment of an ancient riveted metallic bridge based on submodelling of the critical detail

Increasing traffic demands (ie, load intensity and operational life) on ancient riveted metallic bridges and the fact that these bridges were not explicitly designed against fatigue make the fatigue performance assessment and fatigue life prediction of riveted bridges a concern. This paper proposes a global‐local fatigue analysis method that integrates beam‐to‐solid submodeling, elastoplastic of material in local region, and local fatigue life prediction approach. The proposed beam‐to‐solid submodeling can recognize accuracy local stress/strain information accompanying with the global structural effect on the fatigue response of local riveted joints. The fatigue life is predicted based on cumulative damage rule, local strains, and number of cycles with consideration of traffic data, where the relation between the fatigue life and local strain is derived according to the Basquin and Manson‐Coffin law. Besides, the elastoplastic of material is considered. The proposed methodology for fatigue life prediction based on local strain parameter and the Palmgren‐Miner linear damage hypothesis is implemented in a case study of an ancient riveted bridge.     

Fatigue life prediction model for laser clad AISI 4340 specimens with multiple surface cracks

Fatigue test on laser clad AISI 4340 steel specimens show that multiple surface cracks initiate from the clad-toe region due to clad bead overlap features deposited in a raster scan pattern. A fatigue crack growth modeling algorithm capturing the observed fatigue behavior of periodic multiple co-planar semi-elliptical cracks initiating from these features was developed based on crack closure concepts for small cracks to predict the fatigue S–N curve of laser clad AISI 4340 steel specimens. New solutions for stress intensity factor and clad-toe magnification factor (Mk-factor) are presented for surface cracks propagating from the laser clad-toe region. The fatigue life prediction model is able to start from multiple clad-toe surface cracks propagating from the clad-toe region which coalesce into a dominant surface crack or edge crack before final failure. The fatigue life prediction result was compared to the experiment S–N curve test data and gave good agreement.     

Variable amplitude fatigue of adhesively-bonded pultruded GFRP joints

The fatigue behavior of adhesively-bonded pultruded GFRP joints subjected to variable amplitude loading patterns was experimentally investigated. The failure mode of the examined joints was found to be similar to that under constant amplitude loading. The acceleration or retardation of the crack propagation rate due to the load interaction effects was thoroughly investigated by monitoring crack propagation during the variable amplitude loading. The fatigue life of the joints was predicted using classic fatigue life prediction methodology. Existing models for characterizing the fatigue behavior of the examined joints were employed together with the linear Palmgren–Miner’s rule for the prediction of fatigue life. A simple modification was incorporated into the applied methodology to take into account the load interaction effects introduced under the variable amplitude loading. Comparison of the life predictions to experimental data proved that the introduced modification can significantly improve the accuracy of the classic life prediction methodology.     

Application of inverse first-order reliability method for probabilistic fatigue life prediction

A general probabilistic life prediction methodology for accurate and efficient fatigue prognosis is proposed in this paper. The proposed methodology is based-on an inverse first-order reliability method (IFORM) to evaluate the fatigue life at an arbitrary reliability level. This formulation is different from the forward reliability problem, which aims to calculate the failure probability at a fixed time instant. The variables in the fatigue prognosis problem are separated into two categories, i.e., random variables and index variables. An efficient searching algorithm for fatigue life prediction is developed to find the corresponding index variable at a certain confidence level. Numerical examples using direct Monte Carlo simulation and the proposed IFORM method are compared for algorithm verification. Following this, various experimental data for metallic materials are used for model prediction validation.     

Accelerated testing for long-term fatigue strength of various FRP laminates for marine use

The prediction of long-term fatigue life of various FRP laminates combined with resins, fibers and fabrics for marine use under temperature and water environments were performed by our developed accelerated testing methodology based on the time–temperature superposition principle (TTSP). The five kinds of FRP laminates were prepared under three water absorption conditions of Dry, Wet and Wet + Dry after molding. The three-point bending constant strain rate (CSR) and fatigue tests for these FRP laminates at three conditions of water absorption were carried out at various temperatures and loading rates. As results, the mater curves of fatigue strength as well as CSR strength for these FRP laminates at three water absorption conditions are constructed by using the test data based on TTSP. It is possible to predict the long-term fatigue life for these FRP laminates under an arbitrary temperature and water absorption conditions by using the master curves. The characteristics of time, temperature and water absorption dependencies of flexural CSR and fatigue strengths of these FRP laminates are clarified.